DEVELOPMENT OF NANOCRYSTALLINE IRON-CHROMIUM ALLOY … · DEVELOPMENT OF NANOCRYSTALLINE...

DEVELOPMENT OF NANOCRYSTALLINE IRON-CHROMIUM ALLOY BY

MEANS OF SINTERING AND ION IMPLANTATION FOR INTERCONNECT

APPLICATION IN HIGH-TEMPERATURE SOLID OXIDE FUEL CELLS

DENI SHIDQI KHAERUDINI

A thesis is submitted in

fulfilment of the requirements for the award of the

Degree of Master in Mechanical Engineering

Faculty of Mechanical and Manufacturing Engineering

Universiti Tun Hussein Onn Malaysia

NOVEMBER 2011

v

ABSTRACT This research is aimed to develop the nanocrystalline iron-chromium (FeCr) alloys

by two different sintering methods, spark plasma sintering (SPS) and hot pressing

(HP). The sintering temperatures in SPS are designed at 800 and 900 oC; meanwhile

in HP at 1000 oC. The lower sintering temperature in SPS than HP was carried out in

order to obtain the relatively similar in theoretical density of alloy with a minimum

grain growth. The alloy has a potential application as interconnector in solid oxide

fuel cell (SOFC). The beneficial effect of the reactive element by means of

lanthanum (La) into the alloys surface which is introduced using ion implantation is

also evaluated. The study focused on the properties, including thermal expansion,

oxidation behaviour and electrical resistance of the surface oxide scales. Oxidation

testing was conducted at 900-1100 oC for 100 h in laboratory air. Characterizations

by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy

dispersive X-ray spectroscopy (EDS) were carried out before and after each route or

process to investigate the microstructure, phase change, and formation of the oxide

layer. The specific aspects studied were the effects of nanocrystalline structures,

which influenced by the sintering method; and surface treatment through La ion

implantation of chromia-forming alloys may improve their high thermal stability.

The commercially available ferritic steel is chosen as the comparison with other

high-Cr ferritic model alloys. The results revealed that the FeCr alloy prepared by

SPS, to be more effective to retain nanocrystalline and better properties than those

prepared by HP and commercially available ferritic steel. For all types of materials,

the presence of La had no detectable effect on thermal expansion but a major effect

on oxide scale adherence. The results consistently showed that better reduction in

electrical resistance corresponds with excellent oxidation resistance of the alloy. The

performance of FeCr alloy sintered by SPS and implanted by La exhibited the lowest

oxidation and electrical resistance of the oxide scale.

vi

ABSTRAK

Penyelidikan ini bertujuan untuk membangunkan besi-kromium (FeCr) aloi

nanocrystalline dengan dua kaedah pensinteran yang berbeza, spark plasma sintering

(SPS) dan hot pressing (HP). Suhu pensinteran di SPS ditetapkan pada 800 dan 900 oC; sementara itu, di HP adalah pada 1000 oC. Suhu pensinteran yang lebih rendah di

SPS daripada HP telah digunakan dalam usaha untuk mendapatkan ketumpatan teori

aloi yang hampir sama dengan pertumbuhan butiran yang minimum. Ianya

mempunyai potensi aplikasi sebagai interconnector dalam sel bahan bakar oksida

padu (SOFC). Kebaikan penggunaan unsur reaktif iaitu lanthanum (La) ke

permukaan aloi yang diperkenalkan menggunakan kaedah implantasi ion juga dikaji.

Kajian ini tertumpu kepada sifat bahan iaitu pengembangan haba, pengoksidaan dan

penebatan elektrik pada lapisan permukaan oksida. Pengoksidaan ujian dilakukan

pada 900-1100 oC selama 100 jam di ruang udara makmal. Spesimen teroksida

ditentukan dengan menggunakan pembelauan sinar-X (XRD), mikroskop pengimbas

elektron (SEM) dan tenaga penyebaran sinar-X spektroskopi (EDS) yang dilakukan

sebelum dan selepas pada setiap proses untuk mengkaji mikrostruktur, perubahan

fasa, dan pembentukan lapisan oksida. Aspek spesifik yang diteliti adalah kesan

struktur nanocrystalline yang dipengaruhi oleh kaedah sintering; dan rawatan

permukaan melalui implantasi ion La dimana ianya dapat meningkatkan sifat

kestabilan haba yang tinggi. Keluli feritik komersial dipilih sebagai perbandingan

dengan model aloi Cr feritik. Hasil kajian menunjukkan bahawa FeCr aloi

menggunakan kaedah SPS lebih efektif dalam mengekalkan sifat nanocrystalline

berbanding dari yang dihasilkan oleh HP dan keluli feritik komersial. Untuk semua

jenis bahan, kehadiran La tidak memberi kesan pada pengembangan haba namun

memberi kesan yang besar pada pengikatan oksida. Keputusan yang konsisten

menunjukkan bahawa pengurangan rintangan elektrik selari dengan rintangan

pengoksidaan pada aloi. Prestasi FeCr aloi yang disinter oleh SPS dan diimplan oleh

La menunjukkan pengoksidaan dan rintangan elektrik yang terendah.

vii

CONTENTS

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

ABSTRACT v

ABSTRAK vi

CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF SYMBOLS AND ABBREVIATIONS xvi

LIST OF APPENDICES xviii

CHAPTER 1 INTRODUCTION 1

1.1 Background of Study 1

1.2 Problem Statement 5

viii

1.3 Objectives of Study 5

1.4 Scopes of Study 6

CHATPER 2 LITERATURE REVIEW 7

2.1 Solid Oxide Fuel Cell 7

2.2 Interconnect 9

2.3 Potential Interconnect Alloys 11

2.3.1 Iron-Chromium Alloys 12

2.3.2 Other Metallic Materials 15

2.4 Consolidation Processing Route 17

2.5 Sintering 19

2.6 Surface Treatments via Ion Implantation 24

2.7 Thermal Expansion 29

2.8 Oxidation Resistance 32

2.9 Electrical Resistance 37

CHAPTER 3 METHODOLOGY 42

3.1 Starting Materials 42

3.2 Consolidation of Metal Powders to Bulk Shapes 42

ix

3.2.1 Hot Pressing 43

3.2.2 Spark Plasma Sintering 44

3.3 The as-received Commercial Ferritic Alloy 46

3.4 Ion implantation Process for Surface Treatment 46

3.4.1 Samples Preparation 46

3.4.2 Ion Dose 47


3.6 Oxidation Kinetics 49

3.7 Electrical Resistivity 51

3.8 Phase and Microstructural Characterization 53

3.9 Density 54

3.10 Flowchart 55

CHAPTER 4 RESULTS AND DISCUSSIONS 56

4.1 The FeCr powder 56

4.2 Sintered FeCr 59

4.3 Crystallite Size 64

4.4 Density 65

x


4.6 High Temperature Oxidation 70

4.7 Electrical Resistance 95

CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 98

5.1 Conclusions 98

5.2 Recommendations 99

REFERENCES 101

APPENDICES 110

VITA

xi

LIST OF TABLES

2.1 Linear coefficient of thermal expansion for currently used

SOFC materials 30

2.2 Linear coefficient of thermal expansion (measured in air)

between 30 oC and 800 oC as well as between 30 oC and

1000 oC of interconnect materials for SOFC 31

3.1 The chemical composition of the main elements containing

in the investigated commercially available ferritic alloy (wt%) 46

4.1 Crystallite size of the FeCr samples, as received powder and

after the HP or SPS consolidation, and the as received

commercial alloy by using Williamson-Hall Equation Method 64

4.2 Density of the bulk FeCr samples, after the HP or SPS

consolidation and the as received commercial alloy by using

Archimedes Method 65

4.3 CTE values of FeCr samples and as received commercial alloy 69

4.4 Parabolic rate constants (kp) for investigated alloys calculated

from oxidation data 74

4.5 Electrical resistivity for the investigated alloys obtained from

I-V curve measurements 95

xii

LIST OF FIGURES

2.1 Typical SOFC single cell configuration 8

2.2 Schematic of a typical planar SOFC stack 9

2.3 Phase diagram of Fe-Cr system 13

2.4 The three steps of sintering 17

2.5 Powder metallurgy route 19

2.6 Schematic of the hot pressing technique 20

2.7 Structural changes accompanying the preparation of a sintered product 20

2.8 Basic configuration of a typical SPS system 21

2.9 A schematic drawing of the pulsed current that flows through

powder particles 22

2.10 Comparison of the relative densities of the Ti specimen prepared by

SPS and conventional HP sintering as a function of the sintering

temperature 23

2.11 SEM fractured surface of the Ti specimen prepared (a) by SPS at 700 oC

for 5 min and (b) by conventional HP sintering at 900 oC for 1 h 23

2.12 Schematic configuration of the ion implanter system 25

2.13 Areas of layer depth (thickness) of various surface modification

and coating processes 26

2.14 Cyclic oxidation kinetics of the bare and cerium implanted AZ31

at 500C in air 27

2.15 Weight gain vs. time curves of blank and yttrium-implanted 304

stainless steel during oxidation at 1000 C in air 28

2.16 Plots of mass gain vs time during oxidation of blank and

pre-treated GH128 alloys at 1000 oC in air 29

2.17 Different oxidation kinetics 34

xiii

2.18 Kinetics (for 100 h) of coated and uncoated Crofer 22 APU

at 800 oC in air under atmospheric pressure 35

2.19 Cross sectional backscattered electron images of (a) uncoated,

(b) Y/Co-coated and (c) Ce/Co-coated AISI-SAE 430 stainless

steels oxidized for 1000 h 36

2.20 Temperature-dependence of area specific resistance for undoped

and doped Fe–26Cr–1Mo alloy oxidized at 800 oC for 24 h in air

where Pt electrode was used for measurement 40

3.1 Sintering profile in hot pressing process 44

3.2 Sintering profile in spark plasma sintering process 45

3.3 Schematic of Linseis D-8672-SELB dilatometer 48

3.4 Temperature profile used for dilatometer sintering curves 49

3.5 Schematic of thermal cycles applied in this study 51

3.6 Schematic for the resistivity measurement setup 52

3.7 Flowchart of the experiment 55

4.1 The SEM images of the as received mechanically-alloyed

Fe-20 wt% Cr powder (a) 200 magnification and

(b) 1000 magnification 57

4.2 EDS results of the as received mechanically-alloyed

Fe-20 wt% Cr powder 57

4.3 The XRD results of the as received mechanically-alloyed

Fe-20 wt% Cr powder 58

4.4 Hot pressed samples 59

4.5 Spark plasma sintered samples (a) FeCr with SPS sintered

at 800 oC and (b) FeCr with SPS sintered at 900 oC 59

4.6 SEM images for the as consolidated (a) FeCr with SPS

Sintered at 800 oC, (b) FeCr with SPS sintered at 900 oC,

(c) FeCr with HP sintered at 1000 oC, and (d) the as received

commercial alloys 60

4.7 XRD results for the as consolidated samples, (a) FeCr with

SPS sintered at 800oC and (b) FeCr with HP sintered at 1000oC 62

4.8 XRD results for the as consolidated and as received samples,

(a) FeCr with SPS sintered at 900oC and (b) the as received

commercial alloys 63

xiv

4.9 Comparison of CTE values between implanted and

unimplanted for (a) FeCr SPS800 and (b) FeCr HP1000 samples 67

4.10 Comparison of CTE values between implanted and unimplanted

for (a) FeCr SPS900 and (b) commercial alloy samples 68

4.11 Mass gain as function of oxidation time for all the specimens

during oxidation at 900 oC for 100 h in air 73





4.14 The XRD results for the implanted samples (a) SPS800,

(b) SPS900, (c) HP1000, (d) commercial alloy oxidized

at 900 C 77

4.15 The XRD results for the unimplanted samples (a) SPS800,


at 900 C 77



at 1000 C 78



at 1000 C 78



at 1100 C 79



at 1100 C 79

4.20 SEM Surface morphology of the implanted and unimplanted

specimens (a) SPS800, (b ) SPS900, (c) HP1000,

(d) commercial alloy after 100 h oxidation at 900 oC in air 81



xv





4.23 SEM cross section and EDS line scan of the elements for

Implanted and unimplanted (a) SPS800 and (b) HP1000 alloy

specimens after 100 h oxidation at 900 oC 87


Implanted and unimplanted (a) SPS900 and (b) commercial alloy



Implanted and unimplanted (a) SPS800 and (b) HP1000 alloy




Specimens after 100 h oxidation at 1000 oC 90


implanted and unimplanted (a) SPS800 and (b) HP1000 alloy





4.29 Calculated area-specific resistance (ASR) values of the implanted

and unimplanted specimens after 100 h oxidation at 900 oC,

1000 oC, and 1100 oC in air 96

xvi

LIST OF SYMBOLS AND ABBREVIATIONS

Ce - Cerium

Fe - Ferum/Iron

Cr - Chromium

FeCr - Iron-Chromium

Fe2O3 - Iron Oxide

(Fe,Cr)2O3 - Iron Chromium Oxide

Cr2O3 - Chromium Oxide/Chromia

La2O3 - Lanthanum Oxide

LaCrO3 - Lanthanum Chromite

LaB6 - Lanthanum Hexaboride

La - Lanthanum

Se - Selenium

Ti - Titanium

Y - Yttrium

ASR - Area Specific Resistance

At% - Atomic Percentage

CTE - Coefficient of Thermal Expansion

Commercial - The as-received commercial ferritic alloy

DC - Direct Current

EDS - Energy Dispersion X-ray spectroscopy

FWHM - Full Width at the Half Maximum

h - Hour

HP - Hot Pressing

HP1000 - FeCr specimen as ho pressed at 1000 oC

ICDD - International Centre for Diffraction Data

PDF - powder diffraction file

RE - Reactive Element

xvii

SEM - Scanning Electron Microscope

SOFC - Solid Oxide Fuel Cell

SPS - Spark Plasma Sintering

SPS800 - FeCr specimen as spark plasma sintered at 800 oC

SPS900 - FeCr specimen as spark plasma sintered at 900 oC

Wt% - Weight Percentage

XRD - X-Ray Diffraction

xviii

LIST OF APPENDICES

A Calculation of Crystallite Size by Williamson-Hall Equation

Method 111

B Calculation of Density by Archimedes Method 112

C Electrical Current-Voltage (I-V) Plots 113

D List of Publications 117

1

CHAPTER 1

INTRODUCTION

1.1 Background of Study

For decades the energy situation in the world has become more and more critical.

Conventional energy sources are not sufficient to meet the constantly expanding

needs of humanity, so exploration of new energy sources seems to be a challenging

task for the future. One possibility for the alternative to conventional energy

conversion systems is fuel cell development. Fuel cells are generally regarded to be

of central importance for the transformation to the so-called hydrogen economy.

These devices offer the impressive potential of efficient generation of power using

fuel such as hydrogen with essentially no environmentally harmful by-products. As

such, fuel cells have been the focus of many recent research programs (Stover et al.,

1999; Wu & Liu, 2010; Zhu & Deevi, 2003a).

One of the most promising and attention fuel cell systems seems to be Solid

Oxide Fuel Cell (SOFC) because of its potential for becoming an efficient and high

energy-density power generation device (Quadakkers et al., 2003; Steelle, 2001).

SOFC development requires the combination of broad groups of different

engineering branches. One of the tasks is to invent the most suitable materials for all

SOFC components (e.g. anode, cathode, electrolyte and interconnector).

Solid oxides possess sufficiently high ionic conductivity at the elevated

temperatures so that SOFC must operate at the temperature range of 800 - 1000 °C.

The repetitions of single cells constitute the single stack. However, this structure

requires also a mechanical support and a current collector between the different cells

to provide a higher voltage and power in a serial connection. Both properties are

2

provided by the interconnector plate. The availability of a suitable material for the

interconnector is of key importance for SOFC development. This component is

normally a ceramic based material (lanthanum chromite) but there is research trying

to develop new interconnector fabricated with metallic alloys (Stöver et al., 1999;

Tietz et al., 2002). Metallic materials have the advantage of a higher electronic

conductivity, lower cost and easier fabrication than ceramics (Quadakkers et al.,

2003). However, the SOFC operating environment is harsh and many common

metal alloys are not capable of performing adequately over extended periods. This is

especially challenging because the interconnector is exposed to both oxidizing

conditions at the cathode and reducing conditions at the anode. This has led to the

search for a metal alloy with specific physical and mechanical properties and retains

their strength (thermal stability) at the elevated temperatures that could be applied

successfully as a fuel cell interconnects.

The metal alloys have long been considered as potential candidates for high

temperature applications because their high strength to weight ratio. Among metal

alloys in common use, Cr-based alloys and high Cr- ferritic based alloys seem to be

the most promising metallic interconnector that forms protective Cr2O3 layers upon

oxidation at the elevated high temperatures in the air (Quadakkers et al., 2003). This

is due to a rather slow growth rate and a proper electric conductivity of the oxide

scale (Quadakkers et al., 2003). However, development of the particular ferritic

based alloys is a relatively complicated task because the material needs to fulfil

several requirements to be suitable as a SOFC interconnector, which sometimes can

be even contrary. Based on the requirements in respect to oxidation resistance, the

continued growth of oxide scale or oxidation kinetics below 10-14 g2cm-4s-1 is

required, and a value below 10-15 g2cm-4s-1 would be ideal (Antepara et al., 2005),

which can lead to the increasing of electrical resistance or lead to the thermal

expansion mismatch during thermal cycling in both oxidizing and reducing

atmosphere. A large number of ferritic alloys are commercially available in a wide

range of compositions; conversely, it seems that none of them can fulfil all

requirements for the SOFC interconnector (Quadakkers et al., 2003). Therefore, new

FeCr based alloys have recently been developed specifically for SOFC applications.

These materials seem to be sufficiently good for most of the envisaged SOFC

applications; however, it is still necessary to improve their composition to design

3

alloy, which possesses excellent properties during operation in SOFC relevant

atmospheres.

This present study is, in essence, a discussion on designing a nanostrucutred

alloy with tailored properties, entirely in the solid state from metal powder precursor.

However, the problems arise immediately when trying to manufacture the alloy

related to conventional consolidation process, such as in hot pressing (HP). Mainly,

due to longer processing times and higher temperature conditions, some grain growth

in the structure could not completely be eliminated. When thermal energy is applied

to a powder compact, the compact is densified and the average grain size increased.

The full-density compacts with retaining nanometric grain size (nanostructured

alloy), is of essential significance in interconnector SOFC application, which can

produce better properties of alloys, specifically in the reduction the kinetics of

oxidation and not easy to achieve. Therefore, in this work the mechanical alloyed

Fe-Cr powder had been introduced using spark plasma sintering (SPS) technique to

produce the bulk FeCr alloy, since this process is capable of control of producing

many metallic alloys with a perfectly controlled degree of densification and

microstructure with near theoretical density over 99 % at relatively lower sintering

temperature (200 to 500 oC) than temperature used in conventional HP process

(Omori, 2000). In spite of that, this method also cost effective sintering and can be

completed in a short period of approximately 5 to 20 minutes including temperature

rise and holding times (Omori, 2000). In particular, it is essential to compare these

two sintering process, SPS and HP, in order to demonstrate the effectiveness of these

approaches in improving the high temperature properties of the sintered alloy which

would be of significant in practical importance, specifically in SOFC interconnector.

The use of controlled surface modification is a viable alternative to reduce

oxidation rates and extend the useful life of a potential alloy as interconnects in

SOFC. Ion implantation is a physical method for the modification of surface

properties of materials by insertion of accelerated atoms, within the first atomic

layers into solid substrates (Marest, 1998). As a process, ion implantation is widely

used to modify the oxidation behaviour to the surface of the alloys. Modification of

the corrosion behaviour of metal surfaces by ion implantation will allow the

introducing of a controlled reactive element (RE) concentration into the alloys

surface. It has been known that the minor addition of RE, such as La, Y, Ti, Hf, and

Ce, could significantly improve the spallation resistance of these oxide scales under

4

oxidising and/or reducing conditions and inhibits further by isothermal and thermal

cycling oxidation (Cooper et al., 2008). This implantation on the surfaces of the Fe-

Cr alloys may help mitigate chromium volatility, reduce the oxide scale growth rate

of the chromia-forming alloy which can lead to the formation of phase that may

increase the electrical resistance or lead to the thermal expansion mismatch during

thermal cycling. In this research, the investigation, the beneficial effect of La ion

implantation on the oxidation behaviour of alloys also is considered.

The main emphasis of this study was made to investigate the effect of each

process in the high temperature oxidation resistance in the temperature range

required for SOFC application (800 - 1000 °C). The scale formation mechanisms in

the case of the most promising materials were investigated during oxidation times up

to 100 hours and the influence of minor RE addition, specifically La, trough surface

treatment - ion implantation in the alloy was also elaborated. For a better

understanding of the mechanisms of oxidation for Fe-Cr alloy, the available high-Cr

ferritic model alloys were prepared and incorporated into the test program.

Characterisations by using X-ray diffraction (XRD), scanning electron microscopy

(SEM) and energy dispersive X-ray spectroscopy (EDS) was carried out to

investigate the microstructure, phase change, and formation of the oxide layer. The

electrical conductivity of the interconnector is a crucial property for SOFC

application whereby the conductivity of the chromia based oxide scale formed on the

metallic surface during stack operation has to be taken into account in the overall

conductivity value. Nevertheless, the thermally grown oxide scale of the alloy may

overgrow during operating temperature, resulting in scale spallation due to the

thermal stress may occur in the alloys, which induced by the thermal expansion

mismatch between the scale and substrate. Due to these issues, dilatometry studies

were conducted to observe the thermal expansion behaviour of metal alloys which

predict these values as a function of temperature is also the very important

fundamental parameter in SOFC application.

5

1.2 Problem Statement

The identification and fabrication of suitable interconnector materials are a major

challenge in the development of SOFC. Iron-Chromium or FeCr alloys have

received considerable attention as potential interconnector materials due to high

strength and thermal stability. However, the problems arise immediately when trying

to manufacture the nanostructured FeCr alloy whereby steps must be taken to avoid

the grain growth during solidification, which can influence to the improved

properties of alloys, specifically in oxidation resistance and this is not easy to

achieve. Thus, the temperature and time of consolidation have to be restricted at low

value to keep the nanofeatures by using the advance consolidation process - spark

plasma sintering with respect to microstructural and smaller grain size control. On

the other hand, they may fail through loss of strength or gradually deteriorate with

the surrounding atmosphere during SOFC operating temperature. Therefore, the

formation of a stable oxide layer through ion implantation is required to protect the

underlying materials.

1.3 Objectives of Study

The main objective of the present work is to develop the nanostructured FeCr alloy

using sintering and ion implantation for SOFC temperatures environment

application. In order to achieve this, several sub-objectives are withdrawn:

i. To develop nanostructured FeCr alloys by using spark plasma

sintering method in order to reduce and control the grain growth of

alloys.

ii. To investigate the different sintering process, SPS and HP, in

improving thermal stability of nanostrucutred FeCr alloy in terms of

properties: thermal expansion, oxidation behaviour and electrical

resistance.

6

iii. To determine the effect of lanthanum ion implantation on the FeCr

alloys microstructure and oxidation behaviour at 900, 1000 and

1100C.

iv. To compare the as developed FeCr alloy with the commercial alloy in

terms of the investigated properties.

v. To obtain the best method of developing novel FeCr based alloy for

high temperature application, specifically interconnector SOFC

material, based on the results analyzed.

1.4 Scopes of Study

The scopes of this research include the following aspects:

i. Development of nanostructured FeCr alloys by using SPS sintering

method and compared with conventional HP sintering process.

ii. Surface treatment via ion implantation with doses of lanthanum ion of

11017 ions/cm2.

iii. Thermal expansion test by using push rod dilatometer-thermo

mechanical analyzer between room temperature and 900 oC.

iv. Cyclic oxidation test at 900, 1000 and 1100 C for 100-hours

oxidation times.

v. Electrical resistance test of the oxidized sample by using the two point

probe method.

vi. Microstructure and phase analysis before and after the implementation

of ion implantation and cyclic oxidation by using Scanning Electron

Microscope (SEM), Energy Dispersion X-ray Spectroscopy (EDS),

and X-Ray Diffraction analysis (XRD).

vii. Determination of the optimum way of developing FeCr based alloy

with and without lanthanum ion implantation and different

consolidation technique for high temperature application as fuel cell

interconnector based on the results analyzed.

CHAPTER 2

LITERATURE REVIEW

2.1 Solid Oxide Fuel Cell

Solid oxide fuel cells (SOFC) are devices that generate electricity from the

electrochemical conversion of a fuel and oxidant (Minh, 1993). A simplified

configuration and operating principle of an SOFC using hydrogen as fuel is shown in

Figure 2.1. A basic fuel cell comprised of three components: an electrolyte

sandwiched between two electrodes (an anode and cathode). Under operating

conditions, fuel enters and is oxidized at the anode portion of the cell, liberating

electrons. Electrical power is produced as electrons flow from the anode to the

cathode. Oxidant on the cathode side accepts electrons and is reduced to an ionic

species. The circuit is completed by the diffusion of oxidant ions through the

electrolyte. In practice fuel cells are not used individually, but in modular stacks

where a fourth component-an electrical interconnect joins the individual cells. Since

the reactants are often in the gaseous state, the interconnect usually serves to separate

fuel and oxidant as well, thus preventing the fuel and oxidant gases from mixing.

The interconnect is sometimes designated the bipolar plate, or separator,

emphasizing the different polarities of anode and cathode which it connects or the

gas separation, respectively.

8

Figure 2.1: Typical SOFC single cell configuration (Minh, 1993)

For basic hydrogen SOFC, the electrochemical reactions involved are shown

below (Minh, 1993):

Anode: (2.1)

Cathode: (2.2)

Overall: (2.3)

Since the energy stored in the fuel is converted electrochemically and not

through combustion, it is not subject to Carnot cycle limitations, so the energetic

efficiency of a fuel cell is significantly higher (~60%) (Singhal, 2003; Steele, 2001).

If the heat produced by the fuel cell is recaptured, efficiencies can reach ~80%. Fuel

cells also produce much less polluting by products than traditional combustion

processes. Additional advantages of fuel cells previously mentioned include their

modular construction and potential for cogeneration of energy (electrical and

thermal) (Minh, 1993).

The fuel and oxidant could, in theory, be any combination of gases that would

provide the electrochemical reaction required for power generation to occur.

Hydrogen is by far the most common fuel in use. The hydrogen can be in the form

eOHOH 222

2

22 2

21 OeO

OHOH 222 21

9

of pure hydrogen gas, methane, or alcohols. The oxidant most commonly used is

oxygen which is introduced into the SOFC in the form of natural air or as a pure gas.

With these common gases, the fuel cell reaction produces by-products of water

vapour and heat.

The flat-plate or planar type of SOFC has been the subject of intense research

due to its potential for becoming an efficient, high energy-density power generation

device. A schematic of a planar SOFC (Linderoth, Hendriksen, Mogensen &

Langvad, 1996) is shown in Figure 2.2. The structure is comprised of individual

layers of anode, electrolyte, cathode, and interconnect materials. The power capacity

of a single cell is limited. The individual cells can be layered, or stacked, to form

multi-cell structures thereby providing a higher voltage and power generation

capabilities. Therefore, the structure requires a current collector between different

cells to provide a higher voltage and power which provided by the interconnect.

Figure 2.2: Schematic of a typical planar SOFC stack (Linderoth et al., 1996)

2.2 Interconnect

The interconnect is the most complex of the four fuel cell components. The

interconnect serves several vital functions in the SOFC structure. First, it provides

electrical contact between cells allowing the stack to function as a single power

generation unit. In addition, the interconnect keeps the oxidant and fuel gases from

10

mixing by forming a dense, physical barrier between the repeating cells. In certain

designs, the interconnect may also provide mechanical support to the SOFC

structure. The availability of a suitable material for the interconnect is of key

importance for SOFC development.

The traditional material used for the SOFC interconnect is lanthanum

chromites (LaCrO3). This material exhibits a remarkable high electric conductivity

under SOFC operating conditions (Minh, 1993). Lanthanum chromites meet most of

the above requirements and have proven to be suitable for conventional ceramic fuel

cells. However the largest drawbacks for these materials are their high cost and poor

sintering behaviour, which prevents their gas tightness (Minh, 1993). Also, LaCrO3

is ceramic, processing methods are limited which results in limitations as to the

geometry of interconnect that can be fabricated. Because of these weaknesses of

LaCrO3, significant research efforts have been made to find an alternative material.

Metal alloys are widely seen as possessing superior properties compared with

LaCrO3 which offer the potential to increase efficiency of the SOFC. Metals

generally have very high electrical conductivity. Since any decrease in resistance of

the cell would translate directly into increased output, moving to a more electrically

conductive interconnect material could offer significant advantages. Thermal

conductivity for metals is generally higher than for ceramics. Incorporating a high

thermal conductivity material into a SOFC stack would serve to reduce temperature

gradients within the structure that could impact thermal stresses and stack efficiency.

Metals are generally easy to fabricate into a wide variety of shapes, which would

allow designers to fabricate more complex SOFC designs. Also, many metals and

alloys are relatively inexpensive and readily available.

Metallic alloys, however, also bring several negatives along with these

potential benefits. The two main negative aspects of metal alloys for SOFC

interconnects are oxidation of the metal during operation and the potential for

problems arising due to thermal expansion mismatch between the metal and other

SOFC materials. In a broad sense, metals tend to have higher coefficient of thermal

expansion (CTE) compared with ceramics. In terms of the SOFC, this can cause

significant levels of stress to develop when temperature changes. Metals also tend to

oxidize when exposed to the range of temperatures and atmospheres commonly seen

during operation of a SOFC. Excessive oxidation could lead to such problems as

11

reduced efficiency due to higher surface resistance of the interconnect layer. A

discussion of potential interconnect alloys is given later in this section.

In general terms, the properties which a metal interconnect should possess

can be described as follows (Quadakkers et al., 2003; Zhu & Deevi, 2003a):

i) Thermal expansion. The thermal expansion of the interconnect metal should

match with the other materials in the SOFC. It is particularly important that the

alloy match well with the electrolyte, around 10.5 10-6/oC, so that the thermal

stress developed during start-up and shut-down could be minimized.

ii) Oxidation resistance. The alloy must not corrode severely under the operating

conditions of the fuel cell in both anode and cathode atmospheres. The

parabolic rate constant value for interconnect material is required below 10-14

g2 cm-4 s-1, and a value below 10-15 g2 cm-4 s-1 would be ideal. Furthermore, the

oxidation layer of the interconnect should exhibit some degree of electrical

conductivity and should be adherent to the base alloy.

iii) Electrical conductivity. The acceptable area-specific resistances (ASR) value

for interconnect material is considered to be below 100 mΩcm2.

iv) Thermodynamic stability. The alloy must be thermodynamically stable during

processing and operation of the SOFC. Because, the interconnect is exposed to

both oxidizing conditions at the cathode and reducing conditions at the anode.

v) Innocuous to other SOFC materials. The alloy and its oxidation layer should not

contribute to the chemical degradation of other SOFC materials (i.e. chromia

poisoning of the electrode).

For all these reasons, the interconnect materials constitute one of the basic

components for the functioning and long term reliability SOFC operation stacks.

2.3 Potential Interconnect Alloys

Nowadays, interconnect-concept has been developed using metallic alloys. The

majority of alloys currently investigated for fuel cell interconnects are comprised of

various binary and ternary compositions, such as Cr-based or Fe-Cr binary alloys and

12

Fe-Ni-Cr ternary system. Using these types of materials has the advantage of their

lower costs, easy fabrication and machinability and higher thermal conductivity in

comparison with ceramic interconnects. Nevertheless, metallic alloys show the

problem of a large CTE mismatch than that of the ceramic component, causing large

stresses during cell operation. However, one of the most important features using

these materials is that they are able to form protective oxides at the operating

temperature. The formation of these oxides is of vital importance due to properties

like electrical conductivity that shows a large dependence on them. An example can

be found when an alumina forming alloy is used as interconnect. With such alloys

and at the operating temperature, a very protective alumina oxide layer is formed on

top of the surface, but the main inconvenience is that aluminium oxide has a very

low electrical conductivity (10-6-10-8 Scm-1 at 900 oC) compared with other oxides,

specifically chromium oxide (10-2-10-1 Scm-1 at 900 oC) (Zhu & Deevi, 2003a;

Fergus, 2005). For this reason, the majority of interconnect alloy candidate

researched are chromia formers (Yang et al., 2003).

A metal interconnect must exhibit certain specific properties, several of

which were outlined previously. Many materials can be immediately excluded from

consideration based on these performance criteria and known material properties.

For example, platinum would provide an excellent combination of oxidation

resistance and electrical conductivity but the high material cost would be prohibitive

for the economic production of these devices. Similarly, copper would be relatively

inexpensive and highly conductive, but it would melt during the high-temperature

processing of the hybrid stack. The following section provides an overview of

potential alloy systems, highlighting recent work done and the potential each has for

application as a SOFC interconnect. Several reviews on the topic of potential alloys

for SOFC interconnects have been published recently (Linderoth et al., 1996; Zhu &

Deevi, 2003a; Quadakkers et al., 2003).

2.3.1 Iron-Chromium Alloys

The iron-chromium system has long been used as the basis of many engineering

alloys for high-strength, corrosion-resistant applications. Chromium acts as an -

13

phase stabilizer when added to iron because chromium has the same body centre

cubic (BCC) crystal structure as -iron. This result in the suppression of the face

centre cubic (FCC) -phase of iron and creates the so-called "-loop" in the Fe-Cr

phase diagram, Figure 2.3 (ASM Handbook, 1992). At low temperatures, chromium

and iron do not form a complete solid solution due to the presence of the -phase

which has a tetragonal structure and is generally hard and brittle. The equilibrium

phase diagram predicts the formation of -phase for alloys containing greater than 15

wt% Cr at temperatures between 475 and 821 °C. However, heat-treatments at high

temperatures and long hold times or slow cooling rates are required for the formation

of this phase. For a Fe 27 wt% Cr alloy, it was shown that the -phase would

precipitate out of the -phase after holding at 565 °C for 131 days (Smith, 1993). A

Fe-20 wt% Cr alloy, held at temperatures of 600 °C and above, would be within the

-phase field and would therefore never form -phase even after extended high-

temperature exposure.

Figure 2.3: Phase diagram of Fe-Cr system (ASM Handbook, 1992)

Iron-chromium based alloys have been widely investigated for application as

interconnects in fuel cells (Linderoth et al., 1996; Huang, Hou & Goodenough, 2000;

14

Horita et al., 2003a; Mullenberg et al., 2003). Fe-Cr alloys containing between 15

and 40 wt% Cr have been shown to have average CTE values close to that of YSZ

(Linderoth et al., 1996). Oxidation rates of Fe-Cr alloys have been studied at SOFC

operating temperatures in air (Linderoth et al., 1996), wet air, (Mikkelsen &

Linderoth, 2003), and carbon-containing (Horita et al., 2003b) atmospheres.

Linderoth et al. (1996) used commercially provided binary Fe-Cr alloys

ranging in composition from 10 to 60 wt% Cr to study thermal expansion and

oxidation in air. It was shown that the best corrosion resistance was observed from a

Fe 20 wt% Cr sample. The application of a ceria coating on the surface of a bare Fe

40 wt% Cr sample decreased the scale growth at 1000 °C in air by a factor of four

compared with the same alloy without the ceria coating.

Uehara et al. (2003) investigated the impact of small alloying additions to Fe-

Cr alloys containing ~ 20 wt% Cr on the oxidation rate and contact resistance of the

oxide scale. Contact resistance was found to increase with chromium content while

oxidation rate decreased with increasing chromium. The impact of small variations

in Mn, Si, C, and Al additions were not significant in comparison to that due to a

variation in chromium content.

Oxidation of a Fe 22 wt% Cr alloy was studied in wet air and hydrogen at

two water vapour levels by Mikkelsen & Linderoth (2003). It was determined that a

variation in the water vapour content in the oxidizing atmosphere affected oxidation

rate; lower oxidation rates were observed with increased water vapour contents. This

was determined to be due to the fact that wet air facilitated the vaporization of the

chromia from the oxide layer, thereby lowering the observed weight gain of the

sample. In addition, the oxide scale structure was observed to be different for oxides

grown in hydrogen-rich or air atmospheres. Generally, the corrosion/oxidation

resistance is maintained even at high temperature. They are particularly suitable for

applications in aggressive and corrosive environments up to 900°C.

Horita et al. (2003b) studied the oxidation of two commercial Fe-Cr alloys

containing 16 and 22 wt% Cr in a wet methane atmosphere at 800 °C. Results

indicated that a higher chromium-containing alloy would have a slightly lower

oxidation rate in that environment. Electrical conductivity measurements showed

that the 22 wt% Cr alloy had higher conductivity after the oxidation experiment

compared with the 16 wt% Cr alloy. The oxide layers for both alloys were found to

be comprised of Cr2O3 along with Fe-Mn-Cr spinel.

15

Additions of rare-earth elements, specifically Neodymium (Nd) and

Praseodymium (Pr), were shown by Villafañe et al. (2003) to improve the oxidation

resistance of a Fe-Cr alloy in air. Small additions of these elements (~ 0.03 wt%)

were shown to drastically reduce weight gain of Fe 13wt% Cr alloy in air at 800 °C.

It was shown by Ramanathan (1998) that the additions of small amounts of certain

rare earth elements (Y, La) have been well documented in improving the oxidation

protection properties of chromium.

The majority of researches conducted on Fe-Cr based alloys have used

commercially available or supplier-provided experimental alloys. Oxidation

behaviour of the alloy has typically been the main emphasis of research programs

and only passing attention has been given to thermal expansion behaviour and other

properties, such as electrical resistance. When thermal expansion has been

investigated specifically, the published results have not been sufficiently detailed to

allow for an exacting comparison of thermal expansion mismatch between the metal

and electrolyte material. Presenting data in the form of versus temperature allows

for a more complete picture of the material’s behaviour. Therefore, Fe-Cr binary

alloys were developed in the present work from the perspective of finding an alloy

with sufficient properties of oxidation resistance, high-temperature electrical

resistance and thermal expansion.

2.3.2 Other Metallic Materials

Other categories of chromia forming alloys including Ni(-Fe)-Cr base and

Fe(-Ni)-Cr base alloys (e.g., austenitic stainless steels) have a FCC substrate

structure. In comparison to the ferritic stainless steel (FSS), the FCC base alloys, in

particular the Ni(-Fe)-Cr base alloys, are generally much stronger and potentially

more oxidation resistant in the SOFC interconnect operating environment (Fergus,

2005; Linderoth et al, 1996; Yang et al., 2006a). However, the FCC Ni(-Fe)-Cr

base alloys with sufficient Cr for an appropriate oxidation resistance often exhibit a

high CTE, typically in the range of 15.0 to 20.0 10-6/oC from room temperature to

800 oC, and are much more expensive than the FSS. Due to the CTE mismatch,

significant power loss or degradation in performance has been observed during

16

thermal cycling test of stacks using Ni(-Fe)-Cr base alloy interconnects (Wu & Liu,

2010).

The binary Fe-Ni alloys are generally not considered since this system fails to

produce an adherent and protective oxide layer. The oxides of iron (FeO, Fe2O3, and

Fe3O4) form non-uniform layers that spall under fuel cell condition. NiO similarly

does little impede diffusion and protect the alloy. Due to these reasons, little

attention has been paid to this system for interconnects.

Nevertheless, Ni(-Fe)-Cr base alloys may find application as interconnect

materials through the use of innovative SOFC stack and seal designs and novel

interconnect structures. For example, a cladding approach has been applied to

fabricate a stable composite interconnect structure consisting of FCC Ni-Cr base

alloy claddings on a BCC FSS substrate (Chen, et al., 2005a; Chen et al., 2006). The

clad structure appeared to be stable over 1000 hours at 800oC in air and exhibited a

linear CTE close to that of the FSS, but needs further long-term stability evaluation

before its commercial use.

Nickel-chromium alloys have not received significant attention as potential

SOFC interconnect alloys (Yang, et al., 2003) a notable exception generally because

of the high CTE values observed in this system. In addition, these alloys are

generally more expensive than other oxidation-resistant alloys due to the high nickel

contents.

Traditional alloy design emphasizes surface and structural stability, but not

the electrical conductivity of the scale formed during oxidation. In SOFC

interconnect applications; the oxidation scale is part of the electrical circuit, so its

conductivity is important. Thus, alloying practices used in the past may not be fully

compatible with high-scale electrical conductivity. For example, Si, often a residual

element in alloy substrates, leads to formation of a silica sublayer between scale and

metal substrate. Immiscible with chromia and electrically insulating (Kofstad, 1983),

the silica sublayer would increase electrical resistance, in particular if the subscale is

continuous.

17

2.4 Consolidation Processing Route

It is well-known that materials with the same nominal composition can be produced

in different ways, from classical melting and casting, practiced by conventional

metallurgy, via powder consolidation by powder metallurgy methods, combustion

synthesis with thermo-mechanical treatment, etc. Each technological route produces

material having different microstructure, different concentrations and types of

defects and therefore totally different properties. The first step in obtaining high-

performance metal with a homogeneous microstructure and controlled grain size that

meet the requirements of SOFC application is to prepare powders metal with

controlled stoichiometry and small particle size. However, even if a small-size

powder is used, conventional sintering is often unable to provide dense, very fine-

grained metal, due to the high temperatures still required for densification.

Figure 2.4: The three steps of sintering (Assollant, 1993; Kang, 2005)

In solid phase sintering (Figure 2.4), the temperature of the thermal treatment

is slightly above two-thirds of the melting point. Sintering is subject of many

influences: powder characteristics (morphology, dimension of the grains, purity,

etc.), treatment (temperature, pressure, holding time) and atmosphere (vacuum,

18

reducing, oxidizing or inert (Ar, N2)). The sintering process of the solid sample is

considered to be thermodynamically irreversible. It is expressed by a lessening of

the surface energy (free surface of the grains, then surfaces of the open and closed

pores). Three steps are defined during the sintering (Assollant, 1993; Kang, 2005):

i) Formation of a zone of connection between grains, called the bridge or neck of

matter. This phenomenon is activated by diffusion mechanisms, evaporation-

condensation, plastic deformation, etc., and ends when the bridges have been

raised by close to 50% of the grain radii. This step is accompanied by an

increase of ~15% of the density.

ii) Elimination of the residual cavities or pores, the size of which is directly related

to the surface energy. Being inter-connected, they form a continuous porosity,

which diminishes and drives to a compactness of around 90%. Some cavities

being instable will lead to some isolated pores.

iii) The final step corresponds to almost full disappearance of the porosity, giving a

fully dense material. Note that the grains tend to grow in this last step.

To take the advantage of the unique properties of the high performance

nanostructured material the nanometer range powder particles have to be

consolidated nearer to full theoretical density of the material, i.e. after consolidation

nanofeatures should be retained in the densified material. To achieve this it has to

restrict the grain growth or coarsening during densification. Therefore, the

temperature and time of consolidation are to be restricted at low value in order to

achieve smaller grain sizes.

Among the methods reported for activation of the mass transport during the

sintering process, the application of an electrical current through the sample during

heating represents a promising technique for rapid densification of metal at relatively

low temperatures. The most novel and increasingly used method is spark plasma

sintering, which has clear advantages over conventional sintering methods, such as

hot pressing, making it possible to sinter nanometric powder to near full densification

with little grain growth.

This has become increasingly important recently, with the high thermal

stability application in SOFC system and the need to investigate size effects on the

properties in approaching the nanometer scale. Therefore, the two consolidation

19

methods are used for sintering Fe-Cr powder in this work: the SPS and the HP.

Figure 2.5 shows generally the powder metallurgy route which had been explored in

this project.

Figure 2.5: Powder metallurgy route 2.5 Sintering

Sintering is a process of using heat to turn powdered substances into solids without

actually melting the material and is the most common technique for consolidating

powders. During sintering, the pressed powder particles fuse together, forming

metallurgical bonds. Essentially, it is the removal of the pores between the starting

particles, combined with their growth and strong mutual bonding. The process is

carried out by heating up the “green” part at about 70 to 80 % of the melting

temperature, until full strength is obtained within several minutes to hours. The

biggest problem of this technique is the shrinkage which causes cracking and

distortion. There are many methods of sintering a component. The process is

usually divided in four categories: solid-state sintering; liquid-phase sintering,

viscous flow sintering; and transient liquid phase sintering. Overpressure sintering

uses also pressure to accelerate densification. In this work, the mechanically alloyed

Metal Powders

Hot Pressing Spark Plasma Sintering

Secondary & Finishing Operations

20

powders had been sintered in two different vacuum sintering methods: hot pressing

and spark plasma sintering. The schematic of the hot pressing technique is shown in

Figure 2.6, while Figure 2.7 shows the schematics of the structural changes

accompanying the preparation of a sintered product.

Figure 2.6: Schematic of the hot pressing technique (Ashby et al., 2007)

Figure 2.7: Structural changes accompanying the preparation of a sintered product (Ashby et al., 2007)

The spark plasma sintering (SPS) technique is a fast and relatively new

alternative to conventional hot pressing (HP) used for solidification. Several

different materials can be compacted by the SPS technique: metals, composites, and

oxides, nitrates, carbides, mesoporous materials, etc. Even materials which are

considered to be difficult to sinter can be sintered in short times and at relatively low

temperatures to full density. SPS has also recently been used to sinter carbon

nanotubes with copper (Daoush, 2008; Daoush et al., 2009).

21

The SPS configuration is similar to the conventional HP setup. In both cases

the precursor powder (green body) is loaded into a die, usually made of graphite, and

a uniaxial pressure is applied during sintering process to solidify the powder. In the

HP unit the die is heated by heating elements located in the reaction chamber. In

SPS unit there is no external heating element but the die is heated by a pulsed DC

current that goes through the conductive die, i.e. the die serves both as pressure die

and heating element. This means that the sample can be heated from both outside

and inside. The use of a pulsed direct current also implies that the samples are

exposed to a pulsed electric field during the sintering process. The SPS technique resembles HP to a great extent as discussed below. The

benefits of the SPS technique compared to the HP technique can be summarized as:

(i) Rapid heating/cooling rates and short sintering times can be applied; (ii) Higher

pressures can be used, which in turn yields higher densities at lower temperatures;

(iii) The presence of an electric current/field is said to enhance/activate the sintering;

(iv) Most materials can be densified at low temperatures using considerably shorter

sintering times.

Figure 2.8: Basic configuration of a typical SPS system (Tokita, 2006)

22

The basic configuration of a SPS unit is shown in Figure 2.8. It consists of a

uniaxial pressure device, where the water-cooled punches also serve as electrodes, a

water-cooled reaction chamber that can be evacuated, a pulsed direct current (DC)

generator and pressure-, position- and temperature-regulating systems. In an SPS

experiment, a weighed amount of powder is introduced in a die. The die may be

built up with various materials, such as carbon, WC, refractory alloys, etc.

SPS is a pressure-assisted sintering method that is thought be based on

momentary high-temperature spark (and/or plasma, if present) discharges in the gaps

between powder particles at the beginning of the ON-OFF DC current pulses. It is

supposed that the pulsed current propagates through the powder particles inside the

SPS sintering die, as shown in Figure 2.9. The process inventor also claims that the

ON- OFF DC pulse energising method generates: (i) Spark plasma; (ii) Spark impact

pressure; (iii) Joule heating; and (iv) An electrical field promoting material transfer

and diffusion (Tokita, 2000; Tokita 2002; Tokita, 2006).

Figure 2.9: A schematic drawing of the pulsed current that flows through powder particles (Tokita, 2006)

The major interest in this process, when the sintering parameters have been

mastered, is linked to the extreme rapidly of the thermal treatment. Thus, the

consolidation time is greatly decreased from hours, in the case of the conventional

Plasma heating

23

sintering, to few minutes for the SPS process. Moreover, the sintering temperature

can be diminished by a few hundred degrees compared to conventional hot pressing

sintering.

Figure 2.10: Comparison of the relative densities of the Ti specimen prepared by SPS and conventional HP sintering as a function of the sintering temperature (Eriksson, 2007)

Figure 2.11: SEM fractured surfaces of the Ti specimen prepared (a) by SPS at 700 °C for 5 min and (b) by conventional HP sintering at 900 °C for 1 h (Eriksson, 2007)

It was shown by Eriksson (2007) that the densification takes place

significantly much faster in the SPS than that in HP process. Figure 2.10 shows two

typical relative density-temperature profiles used in SPS and HP process, yielding

fully densified Ti specimen with different microstructures. SEM pictures of these

24

samples are shown in Figure 2.11. The average grain size was noticeably different

depending on the sintering method, as shown in SEM micrograph (Figure 2.11). In

conventional HP sintering, the specimen was fully densified after sintering at

temperatures above 900 °C for 1 h. In contrast, when the samples were prepared by

SPS, the relative density of the theoretical value was reached 99% at 700 °C.

Moreover, even 5 min of treatment using SPS at 600–700 °C resulted in a

significantly high density (95 - 99%). A comparison of the two densification curves

obtained from conventional HP sintering and SPS indicates that employing SPS

effective in obtaining fully-densified specimen with a minimum grain growth at a

low sintering temperature by approximately 200 °C. This also shows that SPS

constitute an innovative technique in the field of material sintering and three

distinguishing factors contribute to its enhanced densification compared to

conventional HP process: i) DC current influence, ii) high heating rates, and, iii) the

simultaneous application of pressure.

2.6 Surface Treatments via Ion Implantation

Thermal stability of the metallic interconnect can influence to an efficiency of the

stack such as the increased electrical resistance due to the growth of oxide layer.

There are two general methods used to improve the thermal stability of an alloy;

alloying addition and surface treatment such as by ion implantation. Of these two

methods, only surface treatments – ion implantation are discussed here.

Ion Implantation is a physical method for the modification of surface

properties of materials by insertion of accelerated atoms, within the first atomic

layers into solid substrates. Ionized atoms are made to accelerate and to bombard the

solid surface. As a consequence, there can be distinct modifications to near-surface

microstructure and chemical, physical and mechanical properties which for example

can appear as changes in corrosion behaviour, electrical properties, stiffness,

hardness, wear resistance, friction response, or other surface-region-sensitive

mechanical properties such as fatigue and contact fracture toughness. Nowadays,

this technique is commonly employed for the surface treatment of cutting and

95

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